Subject: Re: [PATCH] fsmonitor: only enable it in non-bare repositories

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 03 May 2021 19:56:35 +0200

On Mon, May 03 2021, Jeff Hostetler wrote:

> On 5/3/21 9:58 AM, Ævar Arnfjörð Bjarmason wrote:
>> On Thu, Apr 29 2021, Johannes Schindelin via GitGitGadget wrote:
>> 
>>> From: Johannes Schindelin <[email-masked]>
>>>
>>> The entire point of the FSMonitor is to monitor the worktree changes in
>>> a more efficient manner than `lstat()`ing all worktree files every time
>>> we refresh the index.
>>>
>>> But if there is no worktree, FSMonitor has nothing to monitor.
>>>
>>> So let's ignore if an FSMonitor is configured (e.g. in `~/.gitconfig`)
>>> and we're running in a repository without worktree.
>>>
>>> Signed-off-by: Johannes Schindelin <[email-masked]>
>>> ---
> ...
>>>
>>> base-commit: 14d50074ff19e68e7a8d718b22d138882087bbc9
>> This is surely a correct fix for now, but wouldn't it in the future
>> also
>> be useful to run it in bare repositories e.g. to be able cache lookups
>> for non-existing loose objects?
>> 
>
> No, the FSMonitor feature only expects data for paths within the
> working directory.  (And is independent of whether the FS change
> data is provided by my fsmonitor--daemon or provided by a hook-based
> provider, such as Watchman.)  The FSMonitor feature uses that data to
> shortcut scans of the working directory.

Indeed, hence "in the future". I'm not suggesting that it'll do anything
useful by watching anything in the .git directory now, but that it might
be an interesting thing to explore.

> There is no interaction with the contents of the .git/objects
> directory and I'm not sure how that would work.

We'd watch .git/objects and .git/objects/{aa..ff}, then when about to
check for a loose object we'd avoid hitting the FS.

I don't know how useful that is post-61c7711cfea (sha1-file: use loose
object cache for quick existence check, 2018-11-12), but e.g. on NFS
this sort of thing still mattered. I had a "bigger hammer" approach with
[1] that ran (and still does, I believe) on a big corporate
installation.

More generally, if you strace .git access during repo operations you'll
find we're doing all sorts of existence checks etc. all the time. Loose
objects, refs, seeing what packs there are (better with the MIDX, but do
we still fall back?) etc. If we had up-to-date inotify/fsmonitor info we
could ask the daemon about it.

1. https://lore.kernel.org/git/[email-masked]/